Thanks for reaching here! This is in our backlog and product team is working on this but right now would not be able to share timeline for this.
Work around suggestion: You may try combining search results from different indexes so that the combined scores reflects the relevance of each doc in the combined results.
The key is around the distribution of terms in each index. As long as the indexes are fairly similar, you should be able to merge the results and sort them by the search score. If different indexes are going to have very different sets of terms, then just merging them might not work as well.
Here is a sample app that shows doing this across multiple search services, but could easily be modified to do it across multiple indexes instead: https://github.com/Azure-Samples/azure-search-dotnet-samples/tree/main/multiple-search-services
Let us know if further query.